1. Field of the Invention
The present invention relates to a thread groove type vacuum pump suitable for use in forming thin films in the manufacture of integrated circuits, semiconductors and the like.
2. Description of the Related Art
Conventional thread groove type pumps, originally called "molecular drag pumps", have been developed for primary use in the free molecular flow region. In such a pump, which, as shown in FIG. 8, includes a thread groove (4) formed in the outer peripheral surface of a rotor (2) rotably disposed in a stator (1) with a minute gap (3) therebetwen, the width of a ridge (5) between adjacent parts of the groove (4) has been set to be large in order to reduce the rate of leakage flow through the gap (3) and to thereby obtain a high compression ratio.
With the recent progress of industries associated with the application of thin films, such as in the manufacture of integrated circuits and semi-conductors, there has been an increasing demand for development of a vacuum pump which is clean, has a high pumping speed and is capable of being used in a pressure range of from about 1 to 1000 Pa.
However, the conventional thread groove type pumps have had the problem that since the width of the thread groove (4) is small because of the large width of the ridge (5), the pumping speed is very low and, accordingly, the pump cannot be used as a vacuum pump for pumping a large quantity of gas in the aforementioned wide pressure range (1 to 1000 Pa.).